Method of preparing crystalline sodium metasilicate



United States Patent "ice 3,532,458 METHOD OF PREPARING CRYSTALLINE SODIUM METASILICATE David B. Kirby, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Sept. 29, 1967, Ser. No. 671,590 Int. Cl. C011) 33/32 US. Cl. 23-110 4 Claims ABSTRACT OF THE DISCLOSURE The present invention concerns an improved method of preparing anhydrous sodium metasilicate (Na SiO which comprises reacting, usually under pressure, a fine, high SiO -containing sand in an excess of sodium hydroxide for a period less than about 60 minutes at elevated temperatures to precipitate an anhydrous metasilicate product which is characterized by a low insoluble impurity content well below about 0.24 percent and typically below about 0.04 to 0.06 percent.

While many processes have been developed and disclosed for the preparation of sodium metasilicate, these processes are either commercially not feasible, they produce excessive amounts of insoluble impurities, or they produce a hydrated material which is less economical to transport. Accordingly, there is need for a reasonably rapid process which will produce high Purity anhydrous sodium metasilicate containing only a very minute insoluble impurity content by a method readily amenable to commercial production conditions and employing relatively inexpensive raw materials.

A primary object of the present invention is to provide an improvement in a method of producing anhydrous sodium metasilicate, the silicate product being characterized by a surprisingly low level of insoluble impurities of, for example, unreacted silica, sillimanite, montmorillonite, and the like. A related object is to provide such a sodium metasilicate product which is anhydrous and has an insoluble impurity level below about 0.04% to 0.06% and typically not exceeding about 0.1%.

The present invention comprises an improvement to a process for making sodium metasilicate, the process being improved upon being disclosed in US. Letters Patent 2,830,876 issued to Mockrin et al. The Mockrin process upon which is based the present improvement comprises first preparing a sodium silicate liquor in which the Na O-to-SiO weight ratio is within the range of 1.6 to 3.0. So-prepared, the liquor is digested at a temperature of at least 140 C. and preferably from 160 to 200 C. Precipitation of crystals of anhydrous sodium metasilicate occurs during heating until the precipitation is substantially complete, which usually requires a heating time of at least about 20 minutes, but not generally requiring a period of over about 2 hours. After digestion, the solution is cooled to a temperature within a range of from about 70 C. to about 120 C., and preferably about 90 C. The starting sodium silicate solution may be obtained and prepared by digesting silica in aqueous caustic in proportions to provide the aforesaid ratios and to provide an aqueous reaction composition wherein the Na O, SiO and water are present in the proportion of from 27.0 to 42.5% Na O, from to 22% $0,, and water, by difference of from 43 to 60 percent. A minimum disclosed insoluble impurity concentration of from 0.24 to 0.44% is obtained in the Mockrin process as described. Now unexpectedly, the Mockrin process has been improved in accordance with the present invention to provide a superior sodium metasilicate product exhibiting surprisingly low insoluble impurity level.

3,532,458 Patented Oct. 6, 1970 Accordingly, the improvement of the present invention comprises: employing, in the Mockrin et al. process, sand of a predetermined purity and classification, the sand being characterized in that it comprises at least 99.8% of SiO and preferably 99.9% SiO and further characterized in that, prior to digestion with sodium hydroxide at least weight percent thereof will be capable of passing a number 325 mesh screen (US. Standard sieve series), with preferably 9495% being so-capable, the balance being capable of preferably passing a number 200 mesh screen.

The sodium hydroxide employed in the present novel process may be, e.g., as a 50 to 73 percent aqueous solution. In any event sufficient caustic must be employed to provide a Na O-to-SiO weight ratio of from 1.6 to 3 and a reaction composition wherein the Na O concentration is within a range of from about 27.0 to 42.5%, there being sufficient excess caustic in the solution to give a mother liquor with at least about 20% NaOH at the end of the reaction.

The anhydrous sodium metasilicate precipitate produced by the improvement of the present invention is generally of large crystalline size and fast settling such as to render separation from the mother liquor a relatively easy matter. Upon separation, such as, by filtration, the precipitate may be washed with a 28% NH, aqueous ammonia solution or, e.g., methanol, then dried at, 110 C. in an air atmosphere to an anhydrous free flowing powder having a high bulk density. An unusually high CO -containing atmosphere during drying tends to raise the insolubles impurity content and therefore should be avoided.

The term insolubles as used herein means and refers to an impurity appearing in the anhydrous metasilicate product and normally comprises one or more insoluble materials such as, e.g., unreacted silica, sillimanite, montmorillonite, complex silicates, and trace amounts of unidentified inorganic materials.

The following examples serve to illustrate various operable and preferred embodiments of the present invention and are not to be construed as limiting the invention thereto.

EXAMPLE I A stirred pressure vessel was loaded with a slurry of 1000 grams of 50% aqueous sodium hydroxide and 267 grams of -325 mesh sand containing 99.9% by weight SiO The vessel was sealed and heated to 200 C. within about one half hour and held at 200 C. for one-half hour during which time a solid of anhydrous sodium metasilicate was formed. The metasilicate was separated from its mother liquor as a wet cake then washed with 28% NH to reduce the NaCl content followed by a methanol wash to reduce the water content, then dried at 110 C. A dry, stable, high purity sodium metasilicate product was obtained containing about 0.048% by weight insoluble impurity.

EXAMPLE II A slurry of 1000 grams of 50% diaphragm cell caustic, saturated with NaCl, was added to 267 grams of Pennsylvania Pulverizing Supersil, 325 mesh sand. This was slurried in a Parr bomb with a mechanical agitator for 10 minutes. The bomb was closed and all valves sealed off. The bomb was put in the heater, whereupon. air and discharge lines were put in place. The agitator and heat were turned on and the run commenced. The reactor was heated to 200 C. whereupon the reaction mass exhibited an autogenous pressure of 238 p.s.i.g.; the temperature was maintained at this point for one hour during which the pressure rose to 258 p.s.i.g. The bomb was cooled quickly to C. and the air line opened. The

discharge line was then opened and 100 p.s.i.g. air was blown into the bomb, forcing the product slurry out into the centrifuge. The centrifuge was turned at 3120 r.p.m. or 800 gs for 10 minutes and the mother liquor collected in a beaker.

The foregoing examples of the present novel improvement show that when critical mesh size siO -containing sand is employed to produce anhydrous sodium metasilicate in accordance with the present invention, an an hydrous sodium metasilicate product is obtained having The specific gravity of the clears, i.e., centrifugate, D a very low and high acceptable insolubles impurity conwas taken and found to be 1.378. The mother liquor was tent compared to that obtained from the Mockrln et al. found to contain 0.978% NaCl, 20.0% N21 O and 7.35% process on which the present invention improves- 510 The present invention may be modified and changed 2 grams f the Wet t if cake was d i d at 110 10 without departing from the spirit or scope thereof and 1t C. and found to weigh 126 gms. which represents a is understood that the invennon 1s only l1m1ted as defined 22.2% moisture loss. The analysis on the cake gave in the ap claims- 0.267% NaCl, 48.17% Na O, and 36.1% SiO The in- I Claim; solubles content was 0.0592%; the product dissolved in I11 21 method of Propariug anhydrous Sodlum Water quite fast to give a clear olution silicate wherein Silica sand is digested aqueous SOdl' 158 grams of the wet centrifuge cake was washed with um hydroxide in a reactor at a temperature of at least about two bed volumes each of 28% NH solution and about C-, the Sand and Sodium hydroxide boiug in methanol. The weight was reduced to 152 gmS. The sufficient Proportions to provide a mixture containlng product was dried at 110 C. and found to weigh 106 from about to about 425% 2 from about grams which represents a 32.9% moisture loss. The to 22% z, and from about 43 to about 60% Water and cake analysis gave 0.269% NaC], 51.5% Na O, and a 2 2 Weight ratio during reaction of from 44.2% SiO and insolubles of 0.017%. It provided a about 1- to about thereby to precipitate anhydrous Clear l i h dissolved in Water sodium metasilicate, the precipitate so-formed being separated from the reaction mixture and washed; the im- EXAMPLE III provement comprising, digesting said sodium hydroxide with a silica sand which is characterized by a SiO con- The general procedure of Example II was essentially tent of at least about 99.8 Weight percent and a particle repeated to prepare various samples of anhydrous sodisize wherein at least 80 percent of said sand particles are urn metasilicate using the improvement of the present incapable of passing -325 mesh sieve, the balance of said vention. Other comparative samples were prepared and sand particles being capable of passing a -200 mesh carried out with the benefit of the present improvements. sieve, whereby the anhydrous sodium metasilicate pro- The results of these sample runs together with the various duced has a lower insoluble impurity level. conditions under which each was run, amounts of start- 2. The improvement of claim 1 wherein at least 94 ing materials, analysis of the mother liquor as to percent percent of said particles of said sand are capable of pass- Na O and anhydrous metasilicate products are presented ing a 325 mesh sieve, the balance being capable of in the following table: passing a 200 mesh sieve.

TABLE Percent Reaction Mother Dry Cake-alter Washing Food in grams Purity, Pres- Weight; Liq., Quartz Temp, Time, sure, of Dry percent percent percent percent percent Sample NaOH Quartz Size C. min. p.s.i.g. cake Ins N820 NaCl NazO SiOz 1 500 267 99.9 per- 200 258 106 0.017 20.0 .269 51.5 44.2

cent min:

Comparative (A) 500 267 98. 225 350 535 .865 22.5 .189 44.7 39.6 500 267 90.8-200 200 60 279 151 .468 21.2 None 50.0 47.4 500 267 99.9325 200 30 264 794 .048 22.0 0.75 57.6 42.5 500 267 99. 0325 200 10 266 670 0.64 21.4 .289 41.2 37.7

Sample 1 shows that by using a 99.9% SiO -contain- 3. The improvement of claim 1 wherein digestion of ing Sand of a fine mesh Silo a very low ins l said sand and sodium hydroxide is carried out at a tembles impurity is obtained in the anhydrous sodium metaperature f f b t C, t bo t 200 C, Silicato- Comparative Sample Shows that when the 4. The improvement of claim 1 wherein digestion is same mesh size sand is used then the sand is 0nly 98.3 carried out at a temperature of f about to SiO even when reacted with lfIaOI-I at a much high about 200 C. and at least 94 percent of the silica sand temperature and twlce the reaction t me, that a very h1gh 55 is Capable of Passing a mesh Sieve. insolubles content results. Comparanve Example (B) on the other hand shows a very high resulting insolubles Ref nc Cit d content when the SiO content of the sand is as high as 99.8% but where the sand mesh size is essentially only UNITED STATES PATENTS as fine as 200+325 mesh. Samples 2 and 3 show the 60 2 330 37 Mockrin et 1 23 11Q'1 low insolubles content which can be obtained when a 2 1 049 4 1959 Erbe et 1 325 mesh 99.93% SiO sand is employed in accordance 3 271 317 9 19 Otrhalek 23 11 1 X with the present improvement, even at low reaction times of 10 minutes.

EDWARD STERN, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,532, 58 Dated 6 October 1970 Invent David B. Kirbv rs in the above-identified patent It is certified that error appea by corrected as shown below:

and that said Letters Patent are here In the Table in columns 3 and h, under the column headed percent Insol delete "0.6 4" and add .06 4 under the column headed percent NaCl delete "0.75" and add .075

Signed and sealed this 8th dag of June 1971 (SEAL) Atte st EDWARD M.FLETCHER,JR. WILLIAM E. SGHUYLER, JR.

Commissioner of Patents Attesting Officer 

